Re: [PATCH ipsec-next v3 3/9] libbpf: Add BPF_CORE_WRITE_BITFIELD() macro

[Andrii Nakryiko]

On Fri, Dec 1, 2023 at 12:24 PM Daniel Xu <dxu@xxxxxxxxx> wrote:
>
> === Motivation ===
>
> Similar to reading from CO-RE bitfields, we need a CO-RE aware bitfield
> writing wrapper to make the verifier happy.
>
> Two alternatives to this approach are:
>
> 1. Use the upcoming `preserve_static_offset` [0] attribute to disable
>    CO-RE on specific structs.
> 2. Use broader byte-sized writes to write to bitfields.
>
> (1) is a bit hard to use. It requires specific and not-very-obvious
> annotations to bpftool generated vmlinux.h. It's also not generally
> available in released LLVM versions yet.
>
> (2) makes the code quite hard to read and write. And especially if
> BPF_CORE_READ_BITFIELD() is already being used, it makes more sense to
> to have an inverse helper for writing.
>
> === Implementation details ===
>
> Since the logic is a bit non-obvious, I thought it would be helpful
> to explain exactly what's going on.
>
> To start, it helps by explaining what LSHIFT_U64 (lshift) and RSHIFT_U64
> (rshift) is designed to mean. Consider the core of the
> BPF_CORE_READ_BITFIELD() algorithm:
>
>         val <<= __CORE_RELO(s, field, LSHIFT_U64);
>                 val = val >> __CORE_RELO(s, field, RSHIFT_U64);

nit: indentation is off?

>
> Basically what happens is we lshift to clear the non-relevant (blank)
> higher order bits. Then we rshift to bring the relevant bits (bitfield)
> down to LSB position (while also clearing blank lower order bits). To
> illustrate:
>
>         Start:    ........XXX......
>         Lshift:   XXX......00000000
>         Rshift:   00000000000000XXX
>
> where `.` means blank bit, `0` means 0 bit, and `X` means bitfield bit.
>
> After the two operations, the bitfield is ready to be interpreted as a
> regular integer.
>
> Next, we want to build an alternative (but more helpful) mental model
> on lshift and rshift. That is, to consider:
>
> * rshift as the total number of blank bits in the u64
> * lshift as number of blank bits left of the bitfield in the u64
>
> Take a moment to consider why that is true by consulting the above
> diagram.
>
> With this insight, we can how define the following relationship:
>
>               bitfield
>                  _
>                 | |
>         0.....00XXX0...00
>         |      |   |    |
>         |______|   |    |
>          lshift    |    |
>                    |____|
>               (rshift - lshift)
>
> That is, we know the number of higher order blank bits is just lshift.
> And the number of lower order blank bits is (rshift - lshift).
>

Nice diagrams and description, thanks!

> Finally, we can examine the core of the write side algorithm:
>
>         mask = (~0ULL << rshift) >> lshift;   // 1
>         nval = new_val;                       // 2
>         nval = (nval << rpad) & mask;         // 3
>         val = (val & ~mask) | nval;           // 4
>
> (1): Compute a mask where the set bits are the bitfield bits. The first
>      left shift zeros out exactly the number of blank bits, leaving a
>      bitfield sized set of 1s. The subsequent right shift inserts the
>      correct amount of higher order blank bits.
> (2): Place the new value into a word sized container, nval.
> (3): Place nval at the correct bit position and mask out blank bits.
> (4): Mix the bitfield in with original surrounding blank bits.
>
> [0]: https://reviews.llvm.org/D133361
> Co-authored-by: Eduard Zingerman <eddyz87@xxxxxxxxx>
> Signed-off-by: Eduard Zingerman <eddyz87@xxxxxxxxx>
> Co-authored-by: Jonathan Lemon <jlemon@xxxxxxxxxxxx>
> Signed-off-by: Jonathan Lemon <jlemon@xxxxxxxxxxxx>
> Signed-off-by: Daniel Xu <dxu@xxxxxxxxx>
> ---
>  tools/lib/bpf/bpf_core_read.h | 34 ++++++++++++++++++++++++++++++++++
>  1 file changed, 34 insertions(+)
>
> diff --git a/tools/lib/bpf/bpf_core_read.h b/tools/lib/bpf/bpf_core_read.h
> index 1ac57bb7ac55..a7ffb80e3539 100644
> --- a/tools/lib/bpf/bpf_core_read.h
> +++ b/tools/lib/bpf/bpf_core_read.h
> @@ -111,6 +111,40 @@ enum bpf_enum_value_kind {
>         val;                                                                  \
>  })
>
> +/*
> + * Write to a bitfield, identified by s->field.
> + * This is the inverse of BPF_CORE_WRITE_BITFIELD().
> + */
> +#define BPF_CORE_WRITE_BITFIELD(s, field, new_val) ({                  \
> +       void *p = (void *)s + __CORE_RELO(s, field, BYTE_OFFSET);       \
> +       unsigned int byte_size = __CORE_RELO(s, field, BYTE_SIZE);      \
> +       unsigned int lshift = __CORE_RELO(s, field, LSHIFT_U64);        \
> +       unsigned int rshift = __CORE_RELO(s, field, RSHIFT_U64);        \
> +       unsigned int rpad = rshift - lshift;                            \
> +       unsigned long long nval, mask, val;                             \
> +                                                                       \
> +       asm volatile("" : "+r"(p));                                     \
> +                                                                       \
> +       switch (byte_size) {                                            \
> +       case 1: val = *(unsigned char *)p; break;                       \
> +       case 2: val = *(unsigned short *)p; break;                      \
> +       case 4: val = *(unsigned int *)p; break;                        \
> +       case 8: val = *(unsigned long long *)p; break;                  \
> +       }                                                               \
> +                                                                       \
> +       mask = (~0ULL << rshift) >> lshift;                             \
> +       nval = new_val;                                                 \
> +       nval = (nval << rpad) & mask;                                   \
> +       val = (val & ~mask) | nval;                                     \

I'd simplify it to not need nval at all

val = (val & ~mask) | ((new_val << rpad) & mask);

I actually find it easier to follow and make sure we are not doing
anything unexpected. First part before |, we take old value and clear
bits we are about to set, second part after |, we take bitfield value,
shift it in position, and just in case mask it out if it's too big to
fit. Combine, done.

Other than that, it looks good.

> +                                                                       \
> +       switch (byte_size) {                                            \
> +       case 1: *(unsigned char *)p      = val; break;                  \
> +       case 2: *(unsigned short *)p     = val; break;                  \
> +       case 4: *(unsigned int *)p       = val; break;                  \
> +       case 8: *(unsigned long long *)p = val; break;                  \
> +       }                                                               \
> +})
> +
>  #define ___bpf_field_ref1(field)       (field)
>  #define ___bpf_field_ref2(type, field) (((typeof(type) *)0)->field)
>  #define ___bpf_field_ref(args...)                                          \
> --
> 2.42.1
>